1. Field of the Invention.
The present invention relates to alarm systems, such as intrusion alarms, fire alarms, etc., and more particularly to "wireless" alarm systems in which information is transmitted from local transmitter units to one or more central receiver units without the aid of wires.
2. Description of the Prior Art.
Alarm systems wherein emergency condition sensors (e.g., for sensing incipient fire, intrusion, flooding, etc.) are distributed about premises to be protected and interconnected with one or more central control units are well-known in the prior art.
As is further well-known in the prior art, such central control units control the operation of alarm signal producing means, such as bells, gongs, buzzers, electronic sounders, and the like. Further, such prior art alarm system central units are commonly provided with delay means for delaying the actuation of said alarm signal producing means until a predetermined time after an emergency condition signal is transmitted to a central receiving unit from one of said emergency condition sensors, and further provided with key-operated disarming switch means whereby a key-holder may disarm the system and thus prevent the emission of said alarm signals by inserting and turning the key during the delay interval provided by said delay means.
It is also known in the art to provide exit delay means, whereby the key-holder may arm, i.e., electrically actuate, the alarm system, and then leave the protected premises without triggering an alarm, provided the key-holder leaves the protected premises within a very short predetermined delay interval.
It is also known in the prior art to reduce the cost and inconvenience attending the installation of prior art alarm systems by providing "wireless" interconnections between groups of emergency condition sensors and the central control unit or units associated with said groups, each such group of emergency condition sensors being physically located in close proximity to each other, e.g., in a single room or wing of a building, whereby they can easily be interconnected with a local transmitter unit by means of light, substantially invisible wires. In these prior art "wireless" alarm systems, all of the local transmitters of the system are connected by wireless link means, e.g., radio waves or carrier signals on power wiring, to a suitable receiver which itself forms a part of the central control unit.
It has been found, however, that both the prior art radio wave and carrier types of "wireless" alarm systems, and especially those of relatively little electronic complexity, and thus low cost, are susceptible to "false alarms", produced when stray electrical signals, e.g., emanated by diathermy, radar, television, and radio equipment, and certain transients on power lines, are "received" by the receiver of the central control unit and interpreted as emergency condition signals.